The calcium-sensitive bioluminescent protein aequorin and related photoproteins will be used to study the regulation of intracellular Ca++ concentration in frog skeletal muscle, and in frog and mammalian cardiac muscle. Results will be compared in some circumstances with those obtained with calcium-selective micro- electrodes. High-gain microscopic image intensification with computer image processing will be used to localize Ca++ in aequorin-injected cells. A major effort will be mounted to resolve the isospecies of aequorin and other photoproteins, and to study their kinetics and Ca++-sensitivity. Particular emphasis in the studies on cardiac muscle will be given to the analysis of the role played by changes in myofibrillar responsiveness to Ca++ in responses of the heart to inotropic interventions, particularly the response to alpha-adrenoceptor stimulation. Systematic analysis will also be carried out of the factors responsible for alterations in the Ca++ transient after changes in muscle length and contraction frequency. In skeletal muscle a variety of experiments are planned to critically test the hypothesis that inositol triphosphate release is centrally involved in excitation:contraction coupling.